There are many occasions in the mining industry, in agriculture and in waste water treatment when it is necessary to separate fine particulate matter from a body of a liquid such as water. This is often achieved by a process of sedimentation or settling. The rate of sedimentation of particles is partly dependent upon their mass and it is therefore advantageous in many cases to add a flocculant to the body of water to induce the fine particles to aggregate or floc together. This increases the mass of the particles and hence their sedimentation rate. As flocculant chemicals are expensive and as there may be adverse environmental effects from excessive flocculant addition it is .desirable to add only sufficient flocculant to the body of liquid to impart the desired rate of sedimentation to the particles in the liquid.
It is known to periodically measure the sedimentation rate of particles in a body of a liquid and to control the rate of addition of the flocculant to the body of liquid in accordance with the rate of sedimentation of the particles. If the particles are not sedimenting fast enough then more flocculant is added. If the particles are settling faster than is required then the addition of flocculant may be slowed down or stopped.
In known devices for the above purpose a sample of the liquid is drawn up into a sample container by the induction of a reduced pressure in the sample container. Once the liquid level in the container has reached a desired level a pinch valve in the lower end of the sample tube is closed and the vacuum applied to the sample container is released. The pinch valve serves to hold the liquid level constant in the sample container until the rate of sedimentation of the particles in the liquid has been determined. The pinch valve is then opened to release the sample from the sample container and to allow it to flow back into the body of the liquid.
It has been considered necessary by those working in the field to use a pinch valve or some similar mechanical valve means to hold the sample in the sample container. This perception is based on the need to hold the level of the liquid in the sample container with absolute certainty. If the level of the sample in the sample container were allowed to drop during the period during which the sedimentation measurement were taking place the accuracy of the measurement would be seriously jeopardised. The use of such a mechanical valve however has a number of disadvantages. It has in fact proven difficult to provide a guaranteed seal in the pinch valve despite substantial efforts over many years to provide a satisfactory mechanical seal. The vibration caused by the opening and closing of the pinch valve have been found to adversely affect electronic control equipment associated with the device. It has also been found that in the field where the valve is required to open and close many times each day for years on end the deterioration of the components of the valve parts present a serious maintenance load.
Australian Patent 528273 describes slurry suspension sampling equipment in which the liquid sample is drawn into a sample chamber and temporarily held at a predetermined level to allow for the measurement of the settling rate of the suspension by a timer means which measures the actual settling period of the suspension between two predetermined levels. After the determination of the settling rate and the consequent adjustment of the flocculent flow control valve, the sample of liquid is discharged from the settling chamber via a discharge pipe located at the lower end of the settling chamber and which is normally held closed by a solenoid operated valve.
The present inventor has found that the presence of a pinch valve or some similar mechanical valve at the bottom of the sample container is, in fact, not necessary. This inventor has surprisingly found that it is possible to hold the sample in the sample container with an acceptable accuracy merely by holding the reduced pressure above the sample for the duration of the period during which the sedimentation rate of particles in the sample is determined, This approach substantially reduces the cost of the device and avoids many of the problems associated with the prior art arrangement.